flask console log

from __future__ import print_function # In python 2.7
import sys

@app.route('/button/')
def button_clicked():
    print('Hello world!', file=sys.stderr)
    return redirect('/')

Here is what the above code is Doing:
1. We’re using the @app.route decorator to tell Flask what URL should trigger our function.
2. When the user goes to the URL /button/, the function button_clicked() is run.
3. The function button_clicked() prints ‘Hello world!’ to the console, and then redirects the user back to the home page.